1. Field of the Invention
This invention relates generally to computer networking systems and more particularly to transmitting data from a local area network over a wide area network.
2. Discussion of the Related Art
Computer networks are widely used to provide increased computing power, sharing of resources, and communications between users. Local area networks (LANs) may include a number of computer devices within a room, building, or site that are connected by high speed data links such as token ring, ethernet, or the like. LANs have traditionally been interconnected by bridges, hubs, and switches. A Wide Area Network (WAN) may link together different LANs. A WAN typically includes packet switches, microwave links, and satellite links. Thus, a network may include several hundred or more interconnected devices (nodes), distributed across several geographical locations, and belonging to several organizations.
LANs are typically used for high speed communications with relatively few nodes, while WANs are used for connecting a vast number of nodes, but generally at a slower speed. Therefore, LAN transport mechanisms and protocols are typically not appropriate for use on a WAN.
FIG. 1 shows an example of an arrangement in which a WAN couples together several LANs. In particular, FIG. 1 shows WAN 10 connecting LAN A, LAN B and LAN C. A LAN/WAN interface 12 provides translation, data formatting, and/or other protocol-related functions so that source and destination computers on LAN A may interface with source and destination computers on LAN B and LAN C. Similarly, LAN/WAN interface 14 allows LAN B to be coupled to WAN 10, and LAN/WAN interface 16 allows LAN C to be coupled to WAN 10.
One method of interfacing a LAN to a WAN is to provide a router-specific interface which substitutes a WAN physical link layer and a proprietary data link layer in each LAN data message, and rate-converts the LAN messages to WAN compatible speeds. Then the messages may be transmitted across the WAN. A drawback to such an approach is that many of the high-capacity networks that provide high-speed communications within the WAN are inflexible and expensive. Additionally, there is very limited network management and maintenance support capabilities, and very little, if any, spare signal capacity within frame structures, in the WAN protocol. Thus, this technique is generally very expensive and slow.
SONET (Synchronous Optical Network) is a standard for a high-capacity optical telecommunications network. It is a synchronous digital transport system intended to provide a more simple, economical and flexible network infrastructure. The Phase 1 SONET standard issued in March 1988, and is defined in "American National Standard for Telecommunications-Synchronous Optical Network (SONET) Payload Mappings", ANSI T1,105.02-1993 draft, which is incorporated by reference.
SONET may also be defined as an octet-synchronous multiplex scheme that defines a family of standard rates and formats. Despite the name, SONET is not limited to optical links. Electrical specifications have been defined for single-mode fiber, multi-mode fiber, and CATV 75 ohm coaxial cable. The transmission rates are integral multiples of 51.840 Mbps, which may be used to carry T3/E3 bit-synchronous signals. The allowed multiples are currently specified as:
______________________________________ STS-1 51.840 STS-3 155.520 STS-9 466.560 STS-12 622.080 STS-18 933.120 STS-24 1,244.160 STS-36 1,866.240 STS-48 2,488.320 ______________________________________
The CCITT Synchronous Digital Hierarchy (SDH) defines a subset of SONET transmission rates, beginning at 155.520 Mbps, as set forth below:
______________________________________ SONET SDH equivalent ______________________________________ STS-3c STM-1 STS-12c STM-4 STS-48c STM-16 ______________________________________
As defined, SONET can be used in loop carrier, local network, and long haul network application areas.
FIG. 2 is a block diagram of SONET elements coupled together in a direct synchronous multiplexing mode. SONET line signals 21 are transmitted across the SONET digital cross-connect system 20. SONET terminal multiplexers 22, 24 receive tributary signals 23, for example from source and destination computers, and provide the SONET line signals.
One advantage of the SONET standard is that individual tributary signals may be accessed within the structure of the multiplexed SONET line signal. For example, SONET add-drop multiplexers 26 and 28 may each provide data from their respective tributary signals 25 onto the SONET digital cross-connect system 20.
The building block of the SONET protocol is a synchronous transport frame 30, shown in FIG. 3. Frame 30 includes transport overhead 34, and synchronous payload envelope (SPE) 32. Although data is transported serially, the frame 30 is typically represented by a two-dimensional map with N rows and M columns; a byte is provided at each row/column intersection. The signal bits are transmitted starting with the byte in the upper left hand corner of the frame 30, followed by the second byte in the top row, etc., until all of the bytes of the first top row are transmitted. The second and subsequent rows follow transmission of the first row in the same manner.
The SPE 32 includes individual tributary signals, and is designed to traverse a SONET network from end to end. The SPE 32 is assembled and disassembled only once, even though it may be transferred from one transport system to another (i.e., between several SONET network nodes) many times on its route through the SONET network.
Some signal capacity is allowed in each frame 30 for transport overhead 34 to provide support and maintenance facilities, such as alarm monitoring, bit-error monitoring, and data communications channels. Transport overhead 34 typically pertains only to an individual transport system and is not transferred when the SPE 32 is transferred between different transport systems.
One method of interfacing a LAN with a WAN, such as SONET, is to use the Asynchronous Transfer Mode (ATM) standard defined by the ATM Forum in the "UNI 4.0" specification. Each ATM cell has an overhead of 5/48 or 10.4%. This is a relatively large amount of overhead, when compared to a typical LAN packet. FIG. 4 shows an ATM cell 40, in which 48 bytes of ATM data 44 are transmitted with a 5-byte ATM header 42. In accordance with the ATM standard, an ATM cell always has 53 bytes.
FIG. 5 shows a SONET synchronous transport frame 49 that includes ATM cells. The synchronous transport frame 49 includes SPE 53 and a transport overhead 48 which includes section overhead 50, pointer 52 and line overhead 51. The pointer 52 is a reference which indexes the starting point of the SPE 53. SPE 53 includes path overhead 54, ATM cells 55A, 55B, 55C, 55D, and 55E, and other data. During times for which there are no ATM cells to transmit, an idle space is provided (see for example idle space 56 between two ATM cells 55D and 55E). In addition, because an integer number of ATM cells may not fit within a given SPE, there may be some empty space 57 (see FIG. 5) following the last ATM cell, or otherwise dispersed within the SPE.
FIG. 6 depicts a known process for converting LAN data to ATM cells for transport over a SONET network. In step 58, a LAN packet is received from a first IAN. The LAN packet is divided into 48-byte segments (step 59) in accordance with the fixed ATM cell size. An ATM 5-byte header is added to each of the 48-byte segments (step 60) to create a number of ATM cells. The ATM cells are packed into a SONET SPE (step 61). The appropriate SONET transport overhead is then added to the SPE (step 62) to create a synchronous transport frame. The frame may then be transmitted over the SONET-compatible network (step 63).
At another SONET multiplexer, the synchronous transport frame is received (step 64). The SONET transport overhead is removed (step 65) to yield the SPE. It is then necessary to unpack the ATM cells from the SPE in order to provide ATM cells (step 66). The ATM header is removed from each of the ATM cells (step 67). Only then may the LAN packet data be reconstructed from the ATM cell data (step 68). The reconstructed LAN packet may then be transmitted onto a second LAN (step 69).
As is evident from FIGS. 4-6, there is a significant amount of overhead which is required to transmit reconstructed LAN data over a SONET network using the ATM standard. This overhead includes not only signal bandwidth, for example the 5-byte ATM header added for each 48 bytes of data, but also significant processing time in order to divide the LAN packet, construct the ATM cells, and ultimately reconstruct the LAN packet.